1. Field of the Invention
The present invention relates to a sheet skew correcting apparatus for a copier, a printer, or the like, and also to an image forming apparatus using it.
2. Background Art
Conventionally, a sheet transporting apparatus which is incorporated in a printer or the like is often provided with a sheet skew correcting apparatus in order to correct a phenomenon of obliquely feeding a sheet, or a phenomenon which is so-called skewing.
FIG. 6 is a front view of a conventional sheet skew correcting apparatus which is incorporated in a printer or the like. The apparatus is configured by: a driving roller (hereinafter, referred to as registration drive roller 1) which, in a sheet transporting path connecting a sheet feeding apparatus to a transferring apparatus, causes the front end of a sheet transported in a sheet transporting path disposed upstream from the transferring apparatus, to abut against the roller, thereby registering the front end of the sheet on the nip line of the roller; a driven roller (hereinafter, referred to as registration idler roller 2) which is similarly disposed; a driving roller (hereinafter, referred to as timing drive roller 4) which is used for transporting the sheet in a sheet transporting path disposed upstream from the registration rollers; a driven roller (hereinafter, referred to as timing idler roller 5) which is similarly disposed; and a sensor 20 which detects the passing of a sheet 22 over a contact point 27 between the timing drive roller 4 and the timing idler roller 5.
The sheet skew correcting apparatus corrects skew of the sheet 22 in the following manner. The timing drive roller 4 and the timing idler roller 5 are rotated in the directions of the arrows shown in FIG. 6, to cause the sheet 22 fed out from the sheet feeding apparatus to abut against a contact point 23 between the registration drive roller 1 and the registration idler roller 2 which are stopping. Thereafter, the timing drive roller 4 and the timing idler roller 5 are rotated in the directions of the arrows shown in FIG. 7 to feed the sheet 22 by an arbitrary distance, whereby the sheet is curved in a range between the registration drive roller 1 and the registration idler roller 2, and the timing drive roller 4 and the timing idler roller 5. The registration drive roller 1 and the registration idler roller 2 are rotated in the directions of the arrows shown in FIG. 7. Thereafter, the curved portion of the sheet is pressed by a buffer paper guide 28 in a direction along which the front end of the sheet 22 is aligned with the contact point 23, whereby the front end of the sheet 22 is aligned with the contact point 23 to correct skew of the sheet 22.
FIG. 8 is a diagram showing the curved state of the sheet 22 in the conventional sheet skew correcting apparatus shown in FIGS. 6 and 7. When the skewed sheet 22 is aligned with the nip line of the registration drive roller 1 and the registration idler roller 2, the curve of the side which first butts against the contact point 23 is larger as indicated by B, and that of the side which lately butts against the contact point is smaller as indicated by C. As a result, the buffer paper guide 28 which presses the curved portion of the sheet 22 presses mainly only the side of the larger curve, so that the force of pressing the front end of the sheet 22 on the side which lately butts is reduced. Therefore, there occurs a phenomenon that the sheet 22 in the range from the curved portion to the contact point 23 is rotated generally by the pressing on the curved portion of the sheet 22 by the buffer paper guide 28, about an intersection 0 of a ridge line a-a′ on the side of the registration drive roller 1, and a ridge line b-b′ connecting the starting points of the sheet 22 on the side of the timing drive roller 4. This phenomenon remarkably occurs particularly in a sheet of high rigidity, such as cardboard.
In the above-mentioned conventional sheet skew correcting apparatus, when, in order to align a sheet with the nip portion of the registration roller pair, the sheet is sent to the registration roller pair by a distance which is longer than the length of the sheet transporting path between the timing roller pair and the registration roller pair to form a curve in the sheet, the force of pressing the front end of the sheet on the side which lately butts against the registration roller pair, against the registration roller pair is reduced. Therefore, a portion which fails to abut against the nip line of the registration roller pair is produced in the front end of the sheet. This causes a problem in that a necessary skew correcting ability cannot be obtained.